Ion implantation systems are used to dope semiconductors with impurities in integrated circuit manufacturing. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam. The ion beam is typically mass analyzed to select ions of a desired charge-to-mass ratio and then directed at the surface of a semiconductor wafer in order to implant the wafer with the dopant element. The ions of the beam penetrate the surface of the wafer to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. A typical ion implanter includes an ion source for generating the ion beam, a beamline assembly including a mass analysis apparatus for mass resolving the ion beam using magnetic fields, and a target chamber containing the semiconductor wafer or workpiece to be implanted by the ion beam.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station.
The mass of an ion relative to the charge thereon (i.e., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway that will effectively separate ions of different charge-to-mass ratios.
In order to achieve a desired implantation for a given application, the dose and energy of the implanted ions may be varied. The ion dose controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dose applications. The ion energy is used to control junction depth in semiconductor devices, where the energy levels of the beam ions determine the degree to which ions are implanted or the depth of the implanted ions within the semiconductor or other substrate material. The continuing trend toward smaller semiconductor devices requires a mechanism that serves to deliver high beam currents at low energies. The high beam current provides the necessary dose levels, while the low energy permits shallow implants.
For shallow depth ion implantation, high current, low energy ion beams are desirable. In this case, the reduced energies of the ions cause some difficulties in maintaining convergence of the ion beam due to the mutual repulsion of ions bearing a like charge. High current ion beams typically include a high concentration of similarly charged ions that tend to diverge due to mutual repulsion. To maintain low energy, high current ion beam integrity at low pressures, a plasma may be created to surround the ion beam. High energy ion implantation beams typically propagate through a weak plasma that is a byproduct of the beam interactions with the residual or background gas. This plasma tends to neutralize the space charge caused by the ion beam, thereby largely eliminating transverse electric fields that would otherwise disperse the beam. However, at low ion beam energies, the probability of ionizing collisions with the background gas is lower. Moreover, in the dipole magnetic field of a mass analyzer, plasma diffusion across magnetic field lines is greatly reduced while the diffusion along the direction of the field is unrestricted. Consequently, introduction of additional plasma to improve low energy beam containment in a mass analyzer is largely futile, since the introduced plasma is quickly diverted along the dipole magnetic field lines to the passageway chamber walls.
Another problem associated with plasma for space charge neutralization is that a plasma source occupies space within the optical element at issue, which then must be configured to be large enough for both the plasma source and the ion beam. Such additional space is costly in terms of power consumption for the optical element and also increases the difficulty of implementation.
In ion implantation systems, there remains a need for a beam containment apparatus and methodologies for use with ion implantation systems including high energy systems, as well as high current, low energy ion beams which may be operated at low pressures, and which provides uniform beam containment along the entire length of a mass analyzer beam guide or other portions of the system.